Page 202 - Pipeline Pigging Technology

Corrosion inspection of the Trans-Alaska pipeline

For example: the ultrasonic transducers are dependent on a reflected echo to
be able to calculate the remaining wall thickness; when a sloped or curved
surface is encountered, the echo is reflected away from the transducer,
causing an invalid or no signal. For this reason, ultrasonic pigs have limitations
or blind areas in bends, dents, and in slack line conditions, due to loss of
couplant. Magnetic pigs have blind areas near girth, longitudinal, and spiral
welds, at expander marks, and in tight bends.
Measurement accuracy also varies between the two technologies. As
noted in the data presented in Table 2, the field verification results of the two
technologies show a small advantage to the ultrasonic technology in this
example. This is probably due to the subjective method of grading or
interpreting the signals which results from the magnetic pig. The reported
corrosion is dependent upon a technician making a judgment on whether or
not a sine-type wave represents corrosion.
Pipetronix has made significant improvements in its magnetic-flux pig
since running the Alyeska pipeline in 1988. The improved features are high-
strength magnets, highly-sensitive and smaller-sized sensor units, and digital
processing of data. In further detail, these enhancements are:

detection capability: expected to increase metal-loss detection from
30% of pipe wall to 10%;
sensing units: are physically reduced in size minimizing blind areas and
girth weld lift off;
data collection and processing: accomplished in digital format which
will enhance analysis.
Alyeska is planning to run the Pipetronix enhanced magnetic flux pig,
called a Magna Scan HR pig, in the summer of 1991. Early experience with
this pig by Pipetronix has exceeded expectations.

Ultrasonic corrosion pig experience

Two successive runs of the NKK ultrasonic corrosion pig in Alyeska's
pipeline offer an opportunity to compare results against known pipe condi-
tions. In 1989, Alyeska exposed 6,300 linear ft of buried pipeline, and in 1990
11,800ft was excavated to inspect the condition of the pipeline and make
repairs where necessary.
At each pipeline excavation, Alyeska had specific procedures which are
prescribed to ensure that all data is collected on the condition of tape, coating,
and pipe wall condition. The tape and coating are removed and the pipe wall

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